# Fat mass and lean body mass (FM)

Body fat applied to weight, and the rest. Lean mass feeds the TDEE engine’s resting-metabolism prior.

Type: body_value · Unit: kg · Domain: Body Composition · Accuracy: Estimate · Importance: Supporting

Also called: fat-free mass, lean mass, LBM, FFM

## In one sentence

Your weight split in two: the kilograms that are fat, and everything else, which is muscle, bone, organs, skin and the water in all of them.

## Why it matters

The two halves answer different questions. Fat mass is the quantity most people want to change. Lean mass is what sets your resting metabolism: the more of it you carry, the more energy you burn doing nothing, which is why Vitality Tracker's energy engine builds its resting-metabolism estimate from lean mass when it has a trustworthy figure. During a diet the pair tells you what you are actually losing; a falling weight with a steady lean mass is fat going, a falling weight with a falling lean mass is muscle and water going with it.

## What moves it

Both are arithmetic on body fat, so both inherit its noise. Lean mass inherits more, because about 73 percent of lean mass is water (Kyle 2004), so a liter of hydration shows up as a kilogram of "lean" overnight. A kilogram of real muscle takes months of training to add.

## How to measure it well

The scale's routine ([*How to use a bioimpedance scale*](https://vitalitytracker.app/knowledge/how-to-use-a-bioimpedance-scale)); a DEXA scan for a calibrated split when you want the real number.

## How Vitality Tracker uses it

The energy engine uses lean mass for its resting-metabolism prior when your scale's body fat has been corrected against a DEXA scan; without that anchor it falls back to an equation that needs only height, weight, age and sex. Lean mass divided by height squared gives the fat-free mass index; the arms-and-legs part of it gives the appendicular skeletal muscle index. Both halves are kept as series per device.

## How far to trust the number

Estimate, with the same within-device consistency as body fat: the trend is believable, the level is the scale's guess until a DEXA scan corrects it.

## Method

Fat mass = weight × body fat %. Lean body mass = weight − fat mass. The founder's reading: 74.87 kg at 8.8% gives 6.56 kg fat and 68.31 kg lean.

## Sources

1. Kyle UG, Bosaeus I, De Lorenzo AD, et al. Bioelectrical impedance analysis, part I: review of principles and methods. Clin Nutr 2004;23:1226–43. [doi:10.1016/j.clnu.2004.06.004](https://doi.org/10.1016/j.clnu.2004.06.004)
2. Vitality Tracker, How Vitality Tracker estimates your energy expenditure, method note v1.0.

## Related

- [Body fat](https://vitalitytracker.app/knowledge/body-fat)
- [Muscle mass and muscle rate](https://vitalitytracker.app/knowledge/muscle-mass-and-muscle-rate)
- [Fat-free mass index](https://vitalitytracker.app/knowledge/ffmi)
- [Appendicular skeletal muscle index](https://vitalitytracker.app/knowledge/asmi)
- [DEXA](https://vitalitytracker.app/knowledge/dexa)
- [The TDEE engine](https://vitalitytracker.app/knowledge/tdee-engine)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/fat-mass-and-lean-body-mass